Cable fault detector

ABSTRACT

The present invention is a apparatus and method for accurately finding a location of a fault in a fiber cable where a cable locating current is leaking to ground. The system includes a sensor body, voltage probes mounted in the body to face the cable, a reference voltage input and a voltage comparator that compares the reference voltage to a measured voltage at the probes. Use of the system may include applying a conductive medium such as a conductive gel between the probes and the cable.

FIELD OF THE INVENTION

[0001] The present invention relates generally to the repair of systemsusing a cable locating current for determining the position ofunderground cables from above ground. More particularly, the presentinvention is an apparatus and method for finding a fault in a cablesheath that is causing the cable locating current to leak to ground.

BACKGROUND OF THE INVENTION

[0002] Many utilities bury pipes and cables (“utility conveyances” or“conveyances”) underground for reasons of both safety and aesthetics.Underground burial often provides protection to utility conveyancesagainst weather and other sources of potential damage. Utilities thatundertake burial of their conveyances usually make extensive efforts toplot the location of each buried conveyance on a map to facilitate itslocation in case of repair or replacement. While a map will indicate thegeneral location of a buried conveyance, more precise locationinformation often becomes necessary, particularly in urban environments.For that reason, most utilities that bury their conveyances undergroundrely on electromagnetic signaling techniques to precisely locate suchconveyances.

[0003] U.S. Pat. No. 5,644,237, issued Jul. 1, 1997 and incorporatedherein by reference in its entirety, describes a principle forelectromagnetic signaling for locating a buried utility conveyance. Tolocate a buried conveyance, a locating tone or signal is applied to ametallic component of the conveyance. In one currently used embodiment,the signal is an AC signal between 80 and 120 volts and having afrequency of from 220 to 440 Hz. As shown in FIG. 1, in the case of acable 100 containing fiber optic bundles 110, a cable locating conductormay be included in the cable as a metallic sheath 120 or a copper tracerwire (not shown) within an insulating sheath 130 surrounding the cable.Using a signal detector of a type well known in the art, a technicianlocates the conveyance by operating the detector above ground to detecta signal generated by current passing through the cable locatingconductor.

[0004] In addition to the AC locating tone, a DC cable locating signalis also sometimes used in conjunction with a vector bar detector inorder to confirm the information from the AC signal. The DC signal in acurrently used system is approximately 80 to 100 volts.

[0005] The cable sheath or insulating sheath 130 surrounds the cablelocating conductor in the buried cable and insulates the conductor fromground. The insulating sheath is applied during the cable manufacturingprocess, and is formed from a durable, flexible insulating material suchas polypropylene. In one commonly used fiber optic cable, the sheath 130has a thickness 138 of approximately ⅛ inch.

[0006] The insulating sheath of an underground cable has been known tocontain cable sheath faults such as fault 135 caused by cracking,puncturing or preexisting manufacturing defects. Those faults may permitwater or other conducting media to penetrate the cable, providing aground path from the cable locating conductor within the sheath. Sheathfaults are most common in locations where a cable is bent to arelatively small radius, such as within manholes or splice boxes, whereextra cable length is commonly wrapped or coiled for later use inrepairs or maintenance.

[0007] A cable sheath fault, once located, can often be repaired byresealing the sheath in the area immediately surrounding the fault. Onecommonly used method for repairing a sheath fault is by wrapping asealing tape around the area that includes the fault, thereby sealingthe leak. The tape may be a poly material and may include an adhesivedesigned to withstand moisture and to adhere securely to the sheath.

[0008] Cable sheath faults that are leaking current to ground may beroughly located using outside plant fault detection equipment as isknown in the art. For example, a drop in the cable locating signalreceived above ground usually indicates the presence of a ground faultin that general area. Those techniques are effective in isolating asheath fault to a certain manhole or splice box. The exact location of acable sheath fault along a cable is, however, very difficult to pinpointeven after it has been isolated to a specific manhole or splice box.Manholes often fill with rainwater, submerging the cable. When a cableis pulled from the water, current flow to ground will often cease,making the fault difficult to pinpoint. In many cases, the fault is notvisible on the sheath because the defect is extremely small or because acrack in the sheath partially closes when the cable is unwound from asplice box.

[0009] One technique frequently used by an outside plant technician isto feel along the cable for any imperfection in the sheath. Often,before the fault is detectable in that manner, the fault mustdeteriorate significantly. That deterioration also makes it impossibleto use the cable locating current to locate the cable underground in theevent of a third party dig alert, cable maintenance, cable repairs, etc.If a technician is not successful in pinpointing the fault eithertactilely or visually, it is not possible to repair the sheath at thefault location. In those cases, entire sections of cable, typicallyhundreds of feet between splices, must be replaced.

[0010] There is therefore presently a need for a method and apparatusfor locating a sheath fault on a cable such as a fiber cable, withsufficient precision to repair the fault without replacing the entiresection. To the inventors' knowledge, there is currently no suchapparatus or method currently employed to satisfactorily accomplish thattask.

SUMMARY OF THE INVENTION

[0011] The present invention addresses the needs described above byproviding a an apparatus and a method for locating on an optical fibercable a fault where a cable locating current is leaking to ground. Oneembodiment of the invention is an apparatus for locating such cablefaults. The apparatus includes a body adapted to be positioned adjacentthe cable, at least one voltage probe mounted in the body and positionedin the body to probe the leaking cable locating current, a referencevoltage input for receiving a reference voltage, and a voltagecomparator electrically connected to the at least one voltage probe andto the reference voltage input, the comparator configured for measuringa test voltage between the reference voltage and the at least onevoltage probe.

[0012] The body may further be adapted to at least partially surround atransverse section of the cable. In that case the at least one voltageprobe may be a plurality of voltage probes angularly spaced around thetransverse section of the cable. The at least one voltage probepreferably presents a conductive surface facing the cable.

[0013] The reference voltage may be ground, or may be a DC voltageapplied to the cable.

[0014] In another embodiment of the invention, a method is provided forlocating on a cable a fault where a cable locating current is leaking toground. The method includes the steps of positioning a voltage probeadjacent the cable, applying a conductive medium between the cable andthe voltage probe, displacing the voltage probe along the cable,measuring a voltage between the voltage probe and a reference voltage,and, based on the voltage, detecting the fault at a position of thevoltage probe along the cable.

[0015] The conductive medium may be water, a water-based paste or a gel.The voltage probe may include a plurality of conductive surfaces facingthe cable. In that case, the step of positioning a voltage probeadjacent the cable may include at least partially surrounding the cablewith the voltage probe.

[0016] The step of displacing the voltage probe along the cablepreferably includes maintaining the probe in a position at leastpartially surrounding the cable. The step of measuring a voltage betweenthe voltage probe and a reference voltage includes measuring a voltagebetween the voltage probe and ground.

[0017] The method may further include the step of applying a referenceDC voltage to the cable. In that case, the step of measuring a voltagebetween the voltage probe and a reference voltage includes measuring avoltage between the voltage probe and the reference DC voltage.

[0018] The method may include the step of sounding an alarm when thefault is detected. An initial step may be included to determine anapproximate position of the fault by determining a position along thecable where an above-ground detectability of the cable locating currentdegrades.

[0019] The step of detecting the fault may include detecting a drop inthe measured voltage, or it may include detecting an increase in themeasured voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1 is a sectional view of an underground cable with a sheathfault.

[0021]FIG. 2 is a view of an apparatus for locating sheath faultsaccording to the invention.

[0022]FIG. 3 is a flow chart illustrating a method for locating sheathfaults according to the invention.

DESCRIPTION OF THE INVENTION

[0023] An apparatus for precisely locating a cable sheath faultaccording to the present invention is shown in FIG. 2. The apparatuscomprises a test set 210 that an outside plant technician may placearound a cable 205, and a voltage comparator 270 for sensing electricalpotential that may be present between the cable 205 and a referencevoltage such as ground 275.

[0024] In a currently preferred embodiment of the invention, the testset 210 comprises a generally cylindrical body 215 that includes a slotor open area 216. The slot 216 is large enough to permit the cable 205to be inserted within the body 215 and placed adjacent an inner surface218 of the body. The body may then be translated by a technician indirections 220, 221 parallel to an axis of the cable 205 to test forsheath defects along a length of the cable. The inner surface 218 of thebody has a sufficiently large diameter to leave a gap 225 ofapproximately 0.5-1.5 inches of clearance between the surface and theexterior of the cable 205. The body 215 is preferably configured to atleast partially surround the cable in a transverse plane, in order tofacilitate moving the body along the cable, and to increase the surfacearea of the cable that is inspected as the body is translated along thecable.

[0025] The body 215 is equipped with at least one voltage sensor such assensors 230 a, 230 b, 230 c fixed to the inner surface 218 of the body215. The voltage sensors present conductive surfaces facing the cable205 in order to establish electrical continuity between the sensors andthe cable. The illustrated embodiment includes three voltage sensors 230that are equally spaced around the circumference of the cable 205 inorder to further enhance continuity. One skilled in the art willrecognize that other sensor configurations may be used without departingfrom the spirit or operation of the invention.

[0026] As discussed above, cable faults are frequently found on cablelengths that have been wrapped or wound within a manhole or splice box.Those environments are frequently filled with rainwater or ground water.That water may provide a conductive path across the gap 225 between theexterior surface of the cable 205 and the sensors 230. If conditions aredry, or if a more sensitive measurement is required, a conductivematerial such as a gel or water based paste (not shown) may be appliedto the cable to fill the gap 225 and provide an enhanced conductivepath.

[0027] The voltage comparator 270 measures electrical potential betweenthe sensors 230 and some reference voltage. In the illustratedembodiment of the invention, the voltage is measured to ground 275. Thatmeasurement is conveniently carried out by a technician because a groundconnection is almost always available at the site.

[0028] In another embodiment of the invention, a reference voltage isapplied to the cable conductive element in addition to the cablelocating tone. The reference voltage may be a DC signal or may be an ACsignal having a different frequency than that of the cable locatingtone. While less convenient than using a ground reference, the use of adelivered reference voltage assures that ambient electrical potentialthat might be present in the local ground does not affect themeasurement. The apparatus of the invention may be configured to useboth reference voltage sources. Other reference voltage sources may beused without departing from the scope of the invention.

[0029] The voltage comparator 270 may be equipped with a visual, audibleor other signal indicating the presence of a fault. Depending on thereference voltage used, the signal may alert the technician to a suddendrop or rise in voltage, or both.

[0030] A method 300 for locating a fault on an optical fiber cableaccording to one embodiment of the invention is illustrated in FIG. 3.The method is typically performed at a location such as a splice box ormanhole where a fault is suspected. The general location may have beendetermined using above ground fault detection equipment. A voltage probeis positioned (step 310) adjacent the cable. A body containing one ormore probes may be placed directly on the cable as shown in FIG. 2, ormay be of a type that must first be opened in order to insert the cable.To improve the conductive path between the probe and the cable, aconductive medium may be applied (step 320) between the cable and thevoltage probe. That medium may be water, a water-based paste, or a gel.The conductive medium may be applied to the probe, to the cable or toboth, and may be applied either before or after the probe is placed onthe cable.

[0031] The voltage probe is displaced (step 330) along the cable inorder to determine the exact location of the sheath fault. As the probeis moved, a voltage is measured (step 340) between the voltage probe anda reference voltage. As noted above, the reference voltage may be groundor may be a voltage provided on the cable locating conductor.

[0032] Based on the voltage, the fault is detected (step 350) at theposition of the voltage probe along the cable. The fault may be signaledto the technician by an alarm or signal indicating a sudden change involtage as detected by the voltage comparator.

[0033] The foregoing Detailed Description is to be understood as beingin every respect illustrative and exemplary, but not restrictive, andthe scope of the invention disclosed herein is not to be determined fromthe Detailed Description, but rather from the claims as interpretedaccording to the full breadth permitted by the patent laws. For example,while the system is described as having a cylindrical body withcircumferentially spaced probes, other shapes and probe configurationsmay be used while remaining within the scope of the invention. Forexample, the body may be toroidal, horseshoe-shaped, or may simply be ahandle for presenting the probes. It is to be understood that theembodiments shown and described herein are only illustrative of theprinciples of the present invention and that various modifications maybe implemented by those skilled in the art without departing from thescope and spirit of the invention.

What is claimed is:
 1. An apparatus for locating on an optical fibercable a fault where a cable locating current is leaking to ground, theapparatus comprising: a body adapted to be positioned adjacent thecable; at least one voltage probe mounted in the body and positioned inthe body to probe the leaking cable locating current; a referencevoltage input for receiving a reference voltage; and a voltagecomparator electrically connected to the at least one voltage probe andto the reference voltage input, the comparator configured for measuringa test voltage between the reference voltage and the at least onevoltage probe.
 2. The apparatus of claim 1, wherein the body is furtheradapted to at least partially surround a transverse section of thecable.
 3. The apparatus of claim 2, wherein the at least one voltageprobe comprises a plurality of voltage probes angularly spaced aroundthe transverse section of the cable.
 4. The apparatus of claim 1,wherein the at least one voltage probe presents a conductive surfacefacing the cable.
 5. The apparatus of claim 1, wherein the referencevoltage is ground.
 6. The apparatus of claim 1, wherein the referencevoltage is a DC voltage applied to the cable.
 7. A method for locatingon a cable a fault where a cable locating current is leaking to ground,the method comprising the steps of: positioning a voltage probe adjacentthe cable; applying a conductive medium between the cable and thevoltage probe; displacing the voltage probe along the cable; measuring avoltage between the voltage probe and a reference voltage; and based onthe voltage, detecting the fault at a position of the voltage probealong the cable.
 8. The method of claim 7, wherein the conductive mediumis water.
 9. The method of claim 7, wherein the conductive medium is awater-based paste.
 10. The method of claim 7, wherein the conductivemedium is a gel.
 11. The method of claim 7, wherein the voltage probecomprises a plurality of conductive surfaces facing the cable.
 12. Themethod of claim 11, wherein the step of positioning a voltage probeadjacent the cable includes at least partially surrounding the cablewith the voltage probe.
 13. The method of claim 12, wherein the step ofdisplacing the voltage probe along the cable comprises maintaining theprobe in a position at least partially surrounding the cable.
 14. Themethod of claim 7, wherein the step of measuring a voltage between thevoltage probe and a reference voltage includes measuring a voltagebetween the voltage probe and ground.
 15. The method of claim 7, furthercomprising the step of applying a reference DC voltage to the cable, andwherein the step of measuring a voltage between the voltage probe and areference voltage includes measuring a voltage between the voltage probeand the reference DC voltage.
 16. The method of claim 7, furthercomprising the step of sounding an alarm when the fault is detected. 17.The method of claim 7, further comprising the step of initiallydetermining an approximate position of the fault by determining aposition along the cable where an above-ground detectability of thecable locating current degrades.
 18. The method of claim 7, wherein thestep of detecting the fault comprises detecting a drop in the measuredvoltage.
 19. The method of claim 7, wherein the step of detecting thefault comprises detecting an increase in the measured voltage.